Skip to main content

Browse the archive

https://github.com/torvalds/linux
05 April 2024, 19:05:47 UTC
Revision bd09d9a35111b6ffc0c7585d3853d0ec7f9f1eb4 authored by Greg Thelen on 30 October 2013, 20:56:20 UTC, committed by Linus Torvalds on 30 October 2013, 21:27:03 UTC 
percpu: fix this_cpu_sub() subtrahend casting for unsigneds
 
this_cpu_sub() is implemented as negation and addition.

This patch casts the adjustment to the counter type before negation to
sign extend the adjustment.  This helps in cases where the counter type
is wider than an unsigned adjustment.  An alternative to this patch is
to declare such operations unsupported, but it seemed useful to avoid
surprises.

This patch specifically helps the following example:
  unsigned int delta = 1
  preempt_disable()
  this_cpu_write(long_counter, 0)
  this_cpu_sub(long_counter, delta)
  preempt_enable()

Before this change long_counter on a 64 bit machine ends with value
0xffffffff, rather than 0xffffffffffffffff.  This is because
this_cpu_sub(pcp, delta) boils down to this_cpu_add(pcp, -delta),
which is basically:
  long_counter = 0 + 0xffffffff

Also apply the same cast to:
  __this_cpu_sub()
  __this_cpu_sub_return()
  this_cpu_sub_return()

All percpu_test.ko passes, especially the following cases which
previously failed:

  l -= ui_one;
  __this_cpu_sub(long_counter, ui_one);
  CHECK(l, long_counter, -1);

  l -= ui_one;
  this_cpu_sub(long_counter, ui_one);
  CHECK(l, long_counter, -1);
  CHECK(l, long_counter, 0xffffffffffffffff);

  ul -= ui_one;
  __this_cpu_sub(ulong_counter, ui_one);
  CHECK(ul, ulong_counter, -1);
  CHECK(ul, ulong_counter, 0xffffffffffffffff);

  ul = this_cpu_sub_return(ulong_counter, ui_one);
  CHECK(ul, ulong_counter, 2);

  ul = __this_cpu_sub_return(ulong_counter, ui_one);
  CHECK(ul, ulong_counter, 1);

Signed-off-by: Greg Thelen <gthelen@google.com>
Acked-by: Tejun Heo <tj@kernel.org>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
1 parent 3017f07
Raw File
Tip revision: bd09d9a35111b6ffc0c7585d3853d0ec7f9f1eb4 authored by Greg Thelen on 30 October 2013, 20:56:20 UTC
percpu: fix this_cpu_sub() subtrahend casting for unsigneds
Tip revision: bd09d9a
Kbuild 
#
# Kbuild for top-level directory of the kernel
# This file takes care of the following:
# 1) Generate bounds.h
# 2) Generate asm-offsets.h (may need bounds.h)
# 3) Check for missing system calls

#####
# 1) Generate bounds.h

bounds-file := include/generated/bounds.h

always  := $(bounds-file)
targets := $(bounds-file) kernel/bounds.s

quiet_cmd_bounds = GEN     $@
define cmd_bounds
	(set -e; \
	 echo "#ifndef __LINUX_BOUNDS_H__"; \
	 echo "#define __LINUX_BOUNDS_H__"; \
	 echo "/*"; \
	 echo " * DO NOT MODIFY."; \
	 echo " *"; \
	 echo " * This file was generated by Kbuild"; \
	 echo " *"; \
	 echo " */"; \
	 echo ""; \
	 sed -ne $(sed-y) $<; \
	 echo ""; \
	 echo "#endif" ) > $@
endef

# We use internal kbuild rules to avoid the "is up to date" message from make
kernel/bounds.s: kernel/bounds.c FORCE
	$(Q)mkdir -p $(dir $@)
	$(call if_changed_dep,cc_s_c)

$(obj)/$(bounds-file): kernel/bounds.s Kbuild
	$(Q)mkdir -p $(dir $@)
	$(call cmd,bounds)

#####
# 2) Generate asm-offsets.h
#

offsets-file := include/generated/asm-offsets.h

always  += $(offsets-file)
targets += $(offsets-file)
targets += arch/$(SRCARCH)/kernel/asm-offsets.s


# Default sed regexp - multiline due to syntax constraints
define sed-y
	"/^->/{s:->#\(.*\):/* \1 */:; \
	s:^->\([^ ]*\) [\$$#]*\([-0-9]*\) \(.*\):#define \1 \2 /* \3 */:; \
	s:^->\([^ ]*\) [\$$#]*\([^ ]*\) \(.*\):#define \1 \2 /* \3 */:; \
	s:->::; p;}"
endef

quiet_cmd_offsets = GEN     $@
define cmd_offsets
	(set -e; \
	 echo "#ifndef __ASM_OFFSETS_H__"; \
	 echo "#define __ASM_OFFSETS_H__"; \
	 echo "/*"; \
	 echo " * DO NOT MODIFY."; \
	 echo " *"; \
	 echo " * This file was generated by Kbuild"; \
	 echo " *"; \
	 echo " */"; \
	 echo ""; \
	 sed -ne $(sed-y) $<; \
	 echo ""; \
	 echo "#endif" ) > $@
endef

# We use internal kbuild rules to avoid the "is up to date" message from make
arch/$(SRCARCH)/kernel/asm-offsets.s: arch/$(SRCARCH)/kernel/asm-offsets.c \
                                      $(obj)/$(bounds-file) FORCE
	$(Q)mkdir -p $(dir $@)
	$(call if_changed_dep,cc_s_c)

$(obj)/$(offsets-file): arch/$(SRCARCH)/kernel/asm-offsets.s Kbuild
	$(call cmd,offsets)

#####
# 3) Check for missing system calls
#

always += missing-syscalls
targets += missing-syscalls

quiet_cmd_syscalls = CALL    $<
      cmd_syscalls = $(CONFIG_SHELL) $< $(CC) $(c_flags) $(missing_syscalls_flags)

missing-syscalls: scripts/checksyscalls.sh $(offsets-file) FORCE
	$(call cmd,syscalls)

# Keep these two files during make clean
no-clean-files := $(bounds-file) $(offsets-file)
back to top